6. FERTILIZER USE

Fertilizer intensity of crop production
Scenarios of fertilizer intensity
Scenarios of fertilizer use

One scenario of global fertilizer use covering the period 1990-2100 has been published by Pepper et al. (1992). This scenario was based on the growth in GDP, assuming that higher incomes will drive the fertilizer use upwards. The scenario was later used by Alcamo (1994). Contrary to Alcamo (1994), in the projections of Alexandratos (1995) the use of fertilizers is driven by crop production, and fertilizer use is estimated with crop- and land-class-specific fertilizer response curves. As discussed in Chapter 2, the method applied by Alexandratos (1995) to derive projections of fertilizer use is not appropriate for the level of aggregation in this study. Extensive literature studies exist on the subject of plant nutrition, fertilizer-yield relations, and economics of fertilizer use. It is yet very difficult to derive yield response curves for countries or regions. The major reason is that the use of nutrients is not uniform. In particular in developing countries only a minor group of farmers use synthetic fertilizers, while the majority produces at a subsistence level based on crop rotation, recycling of crop residues, organic wastes and animal excreta. Correlation of statistical data of country or regional averaged fertilizer application rates with the average crop yields is therefore not appropriate, although there have been some attempts at it (IFDC, 1992). Another reason why response functions cannot be used for long-term scenarios is the uncertainty about nutrient requirements to sustain future high-yield crops.

Fertilizer intensity of crop production

A simple model was developed to describe the relationship between the total biomass production and the fertilizer intensity, analogous to the feed intensity in livestock production. The fertilizer intensity is the fertilizer input (expressed as the amount of N + P₂O₅ + K₂O) as a fraction of total biomass production. The fertilizer intensity is not identical to fertilizer efficiency of nutrient intensity. Only synthetic fertilizers are considered. Nutrient inputs from animal excreta, biological N fixation, crop residues, precipitation and other sources of nutrients, are omitted, so that the total nutrient input used for crop production may be underestimated. The fertilizer intensity was correlated with the total biomass production from all crops per unit area. Several methods to express total biomass were tested; these are:

· The sum of the harvested biomass of all crops, including unmilled rice (Figure 3a), which does not yield a satisfactory correlation coefficient because of regional differences in the composition of the biomass. For example, Latin America has a fair amount of sugar cane, and in Sub-Saharan Africa root and tuber crops are important. Root and tuber crops and sugar cane produce much more biomass at lower nutrient intensities that other crops such as cereals.

· The sum of harvested biomass from all crops, except oil crops, sugar cane and beet (for which oil and sugar equivalents were used, respectively), which yielded a somewhat better correlation (Figure 3b).

· The sum of the harvested biomass of cereals, including unmilled rice, pulses, the category "other crops", 0.10 x the root and tuber crops and plantain production, oil and sugar equivalents (Figure 3c).

· As the third method, without the category "other crops" (Figure 3d).

FIGURE 3
Relationship between overall yield (Y) and fertilizer intensity (FI) for seven regions covering all the developing countries for 1961/63,1969/71,1979/81,1988/90 and 1989/91; Data represent

a) total harvested biomass;

b) total harvested biomass, sugar and oil;

c) total harvested biomass for cereals, pulses and "other crops" + sugar + oil + 0.10 x roots/tuber/plantain;

d) as c, but excluding the group "other crops";

e) as c, plus data for the U.S.A., former U.S.S.R., Europe, all developed countries, and the world;

f) the curves used to compute the high (top curve), medium (middle curve) and low (bottom curve) fertilizer scenario.

The best correlation was found for the data presented in Figure 3c. Regression analysis of the data set yielded r² for both quadratic and linear functions of close to 0.8 (Figure 3f). The fertilizer intensities vary from one region to another, reflecting differences in the mix of agricultural products, and differences in crop production systems, incorporation of legumes in rotations, recycling of organics and management of animal excreta. Nevertheless it is surprising that the data set shows such a coherent pattern, in which apparently the production becomes more dependent on inputs from synthetic fertilizers at increasing yield levels (Table 14).

TABLE 14
Overall yield (Y) and fertilizer intensities (FI) for the developing regions, and other countries and regions

Region		'61/63	'69/71	'79/81	'89/91	Correction factora
All dev'ping incl. China	yb	1.3	1.6	2.0	2.5
	FIc	6	14	28	36
All dev'ping excl. China	Y	1.3	1.4	1.7	2.1
	FI	6	13	25	31
East Asia	Y	1.7	2.0	2.5	2.9
	FI	10	14	22	33	0.8
China & C.P. Asia	Y	1.5	2.2	3.0	4.0
	FI	6	15	35	46	0.9
South Asia	Y	1.1	1.3	1.5	2.0
	FI	3	12	23	36	1.1
Middle East	Y	1.5	1.7	2.3	2.6
	FI	4	14	32	34	1.0
North Africa	Y	1.8	2.1	2.2	3.0
	FI	13	20	34	29	0.7
Sub-Saharan Africa	Y	0.8	0.9	1.0	1.1
	FI	2	5	10	10	1.1
Latin America	Y	1.7	1.9	2.1	2.4
	FI	9	17	31	29	1.0
USA	Y		2.7	3.2	3.7
	FI		57	56	51
Europed	Y		3.1	3.7	4.3
	FI		70	76	56
Former USSR	Y		1.4	1.4	1.8
	FI		46	81	85
Developed countries	Y		2.3	2.7	3.1
	FI		61	69	60
World	Y		1.8	2.2	2.7
	FI		35	46	46

^a Correction factor used to adjust the results of the equation for the medium scenario for 1990 shown in Figure 3 to achieve consistency with the 1990 data. In some cases additional adjustments were needed to scale to the AT2010 projection.

^b Overall yield, Y (ton/ha) = P / A, where P = the sum of the production of cereals (incl. unmilled rice), pulses, 0.10 x starchy roots production, production of vegetable oils, sugar and other crops (see Appendix 1) in 1000 ton; A = total harvested area in 1000 ha.

^c Fertilizer intensity, FI (in ton/1000 ton) = NPK / P; NPK = the total amount of NPK use in ton (N + P₂O₅ + K₂O).

^d The value of FI for Europe, as calculated from the total fertilizer use, may be incorrect because in this region some of the synthetic fertilizers is applied to grasslands (FAO/IFA/IFDC, 1994).

There are important differences between developing and developed regions (Figure 3e). In most developed regions there is a decrease from about 1980 or earlier (Table 14). In particular the USA has shown a strong decrease in fertilizer intensity with rising yield levels. This may have been caused by an increase of crop rotation with soybeans or other legumes. The crop planted after the legume benefits from the nitrogen in the residues and soil. The overall result of this development is a countrywide increase in the synthetic fertilizer efficiency.

Scenarios of fertilizer intensity

At the same overall yield level the fertilizer intensities are different in the various developing regions¹. However, the increase rate of the fertilizer intensity at increasing yield levels in general is very similar. If the data from the former USSR are excluded, the highest fertilizer intensity is currently seen in Europe. Within Europe the variation is high. For example, the fertilizer intensity in the Netherlands is about 78. European fertilizer intensities may in reality be much lower, since part of the synthetic fertilizer is applied to grasslands.

¹ The contribution to the nutrient input from synthetic fertilizers in China is about 65%, the remainder coming from animal and human excreta, residues, wastes, etc. Including all sources of nutrients would cause an increase of the Chinese "nutrient intensity" to perhaps a level comparable to that in the Netherlands.

In general, the efficiency of nutrient inputs decreases at increasing yields of a specific variety (FAO, 1981; Jauregui and Sain, 1992; Tisdale et al., 1993). However, with improving technologies, the efficiency of synthetic fertilizers use can be increased (Tisdale et al., 1993), and with existing technology it can be made more efficient by, for example, introducing leguminous crops into the rotations or by recycling crop residues. Finally, the use of synthetic fertilizer can be substituted by animal excreta. All these aspects may cause variation in fertilizer intensity between regions.

The scenarios are based on the assumption that the fertilizer intensity reaches a maximum at a general overall yield of 5 ton/ha. This yield level is exceeded in only a few regions in the medium scenario. The assumption of a maximum intensity does not mean that nutrient inputs are bound to a maximum. Using a maximum intensity indicates that in the future the fertilizer use efficiency needs to be increased along with crop yields. As will be demonstrated below, this does not imply constant application rates per unit area. Instead, the application rates per unit of product, which is by definition the fertilizer intensity, are constant.

Different values for the maximum fertilizer intensity were used in the scenarios. The maximum value for the medium fertilizer scenario is the current average fertilizer intensity in Europe of 56 ton NPK/1000 ton, approximately equal to that in all developed countries, achieved at a fairly high yield level. For the high-fertilizer scenario variant a level of 66 ton NPK/1000 ton was assumed; this fell between the European and The Netherlands level. In the low fertilizer scenario variant the maximum intensity is equal to the current world average fertilizer intensity of 46 ton/1000 ton. The latter intensity is close to that of China, a country known for its intensive use of animal and human excreta as fertilizer. All fertilizer scenarios are combined with the medium yield scenarios.

Assuming that crop yields will increase and that fertilizer intensities are constrained, functions describing the fertilizer intensity must have a decreasing slope. Here, quadratic functions were selected with a maximum at yields of 5 ton/ha. At higher yields the fertilizer intensity is constant. Based on the three maxima for the fertilizer intensity, three types of curves result (Figure 3f).

TABLE 15
Synthetic fertilizer use for three scenarios for the period 1960-1990, and the required nutrients for crop production for developing countries, developed countries and the world

			'61/63	'69/71	'79/81	'89/91	2010	2025	2050	2075	2100
ALL DEVELOPING INCL. CHINA
	Total NPKa	FMc	4	14	38	65	123	169	225	261	288
		FHc					147	200	265	308	339
		FLc					100	138	184	214	236
	NPK use/hab	FM	8	22	57	89	147	191	253	303	327
		FH					177	227	298	357	385
		FL					120	157	207	249	269
ALL DEVELOPING EXCL. CHINA
	Total NPK	FM	3	9	22	36	77	112	161	193	217
		FH					93	133	190	228	256
		FL					63	92	132	159	178
	NPK use/ha	FM	7	19	43	64	118	164	232	289	314
		FH					142	195	274	341	370
		FL					96	134	190	237	258
DEVELOPED
	Total NPK			55	76	72
	NPK use/ha			134	174	176
WORLD
	Total NPK			68	114	137
	NPK use/ha			65	100	121

^a Total NPK = total use / requirements in million tons N + P₂O₅ + K₂O

^b NPK use/ha is the use/requirement in kg N + P₂O₅ + K₂O per hectare of harvested land.

^c FM, FH, LF = fertilizer scenarios; FM = medium variant, FH = high variant and FL = low variant of the medium production scenario.

The functions were used to reproduce the fertilizer use for the developing regions for 1990. The exact position of the curves shown in Figure 3f is different from the data for most regions. However, the shape and slope of the curves are similar to the development of fertilizer intensity for individual regions. Therefore, the fertilizer consumption can be reproduced using the same function with region-specific adjustments, in which the shape of the curve and at the same time the maximum corresponding to the scenario are maintained. After this adjustment for some regions a further adjustment is required for the medium scenario to reproduce the AT2010 forecast (Table 14). After 2010 the scalars gradually approach the value of 1, i.e. their effect decreases with time.

Scenarios of fertilizer use

The results for the developing countries as a whole indicate considerable increase in both the total fertilizer use and the fertilizer requirements (Table 15). By the year 2025 the nutrient requirements in the medium scenario even exceed the current fertilizer application rates observed in the developed countries. The strong increase in nutrient requirements is caused by the crop yield increases. The nutrient requirements per hectare increase to values similar to the current EC fertilizer application rate of about 300 kg ha^-1. In some individual regions the required NPK rates even increase to higher values than the current EC fertilization rate (Appendix 18). The environmental consequences of these scenarios in terms of atmospheric emissions of ammonia and nitrous oxide will be discussed in Chapter 8.

It must be noted that the data set used for this analysis is very limited. It may be useful to repeat the study of fertilizer intensity on the basis of country data to better understand the fertilizer intensity. The weakness or greatest uncertainty of the model is the constraint on the fertilizer intensity, which is not allowed to exceed a certain arbitrarily chosen ceiling. In addition, the functions used do not describe the accumulation of stocks. For example, the required phosphorus inputs may decrease with continued fertilization (Van Duivenbooden, 1995).